1. Field of the Invention
The present invention relates to a printing plate for fabricating an alignment film for use in a liquid crystal display (LCD), and more particularly to a method for cleaning a printing plate for fabricating an alignment film that can prevent in advance residuals from solidifying, and an apparatus for cleaning the printing plate applied for such a method.
2. Description of the Related Art
Recently, LCD devices are widely used for flat panel displays. Especially, the smaller, lighter and less power consumptive characteristics make the LCD devices considered as one of the most leading display devices for replacing a cathode ray tube (CRT).
Generally, an LCD device is structured to have two glass substrates with the liquid crystal material injected therebetween. Liquid crystal molecules are arranged in a specific direction and scattered dynamically according to an electrical signal so that the amount of light transmission of the device can be properly controlled. Here, the molecules of the liquid crystal display are required to be arranged in a specific direction to assist the optical function of the LCD. Generally, molecules of the liquid crystal material align themselves locally. Thus, in a conventional manufacturing process, an organic high polymer film directly contacting the molecules of the liquid crystal material is formed on an indium tin oxide (ITO) electrode in order to align the molecules of the liquid crystal material in a specific direction. Here, the organic high polymer film is generally called an alignment film.
Recently, the alignment film is mainly made up of a polyimide resin formed of a polyamide acid and a polyimide. This is because the polyamide acid and polyimide have a high degree of heat resistance and stability, and are easy to deposit and are good at controlling an alignment.
Meanwhile, in forming an alignment film, various methods, such as a spray method, a dip method and a printing method, can be used. Recently, the printing method, for example, a flexo printing method, is widely used for mass productions.
In the flexo printing method, liquid material for an alignment film such as polyimide solution is supplied by a dispenser and passes between a doctor roll and an anilox roll, moving to a printing plate rolled onto a printing roll. Here, a plurality of projections are formed on the surface of the printing plate, and intaglios are defined between projections so as to accommodate the supplied liquid.
Then, the printing plate accommodating the liquid at each projection and intaglio rotates as the printing roll rotates. As rotating, the printing plate contacts a glass substrate moving beneath the printing plate, and forms a thin alignment film on the surface of the glass substrate.
Such a method for forming an alignment film is disclosed in the U.S. Pat. No. 5,533,446 entitled "Thin film forming apparatus and thin film forming method" and the U.S. Pat. No. 5,755,883 entitled "Roll coating device for forming a thin film of uniform thickness".
When the alignment film is all deposited onto the glass substrate, the printing plate is wiped out for another use.
First, the surfaces of the projections and intaglios of the printing plate are wiped out with a soft cloth dipped in solvent. Thus, the polyimide remaining on the surfaces of the projections and intaglios is dissolved by the solvent and wiped out.
Subsequently, the surfaces of the projections and intaglios are wiped out again with a cloth dipped in a volatile solution such as acetone or methyl alcohol. Thus, the solvent remaining on the surfaces of the projections and intaglios are evaporated and removed.
After such a cleaning process, the remaining polyimide or solvents are finally removed by another cleaning solution.
The cleaned printing plate is kept for a certain period in a dark room, and used again when needed.
However, the conventional cleaning method has some serious problems.
The used printing plate manually cleaned by the operator is used again in the next printing process. If the polyimide remaining after the first cleaning step contacts the volatile solution in the second cleaning step, the polyimide remaining at the surfaces of the projections and intaglios chemically reacts to the volatile solution, forming solid residuals.
The solid residuals degrade a uniformity of the alignment film deposited onto the glass substrate.
Moreover, the solid residuals fallen onto the glass substrate may create a plurality of pinholes on the surface of the glass substrate when pressed by the rotating printing roll.
The pinholes obstruct smooth display of the image and significantly degrades an overall printing quality.
Manual removal of the polyimide remaining on the surfaces of the projections and intaglios is also dangerous, since the solvent for removing the polyimide is known as extremely harmful to the human body.
In addition, the manual removal performed whenever a certain printing process is finished degrades the operator's efficiency significantly.